Matt Springer is a graduate student of physics at Texas A&M university. He is also an occasional writer and tinkerer, and he is probably too curious for his own good.
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In physics, as in all mathematical disciplines, we have a lot of need for equations. Those equations contain letters which represent various numbers or quantities. We have plenty to choose from, with the letters both Latin-derived and Greek-derived. Even so that's only a couple dozen letters, and there's a lot of things to represent. E might be energy, or electric field. C might be the speed of light, or just a generic integration constant. But I bet the worst offender if terms of being used for different things is the nondescript and unassuming letter k. Offhand:
k - the spring constant in Hooke's Law.
kb - Boltzmann's constant
k - the Coulomb force constant, k = 1/(4 π ε0)
k - another name for the z direction
k - (with a hat over it), the unit vector in the z direction
K - Kelvin degrees
K - the metric prefix for kilo
k - the wave vector
k - the wavenumber (the magnitude of the above wave vector)
k - a generic (usually integer) constant, after m, n, and l are already used.
k - a Latin index running over 1,2, and 3 for tensor notation in electrodynamics, general relativity and other fields.
K - kinetic energy
KJ - the Josephson constant
K - one of several variants of K mesons in particle physics
Are the more? Almost certainly, this is just what I can remember in 5 minutes of thinking. Maybe it's time for some new letters.
Physical Science
Comments
A couple of corrections:
Kelvin isn't measured in degrees, so saying K stands for Kelvin degrees in incorrect. It's just Kelvin (or possibly Kelvins).
The SI prefix for 1000 is not K, but k.
Posted by: Blaise Pascal | October 27, 2008 10:11 AM
I have been outposted by a speedier pedant. I was leaping to the keyboard over the k = 10^3 issue...
Posted by: andy b | October 27, 2008 10:23 AM
k is often used for thermal conductivity as well.
Posted by: asad | October 27, 2008 10:24 AM
And strikeouts in baseball. Not sure if that counts.
Posted by: Zifnab | October 27, 2008 10:37 AM
Also, k is used for the dielectric constant!
Posted by: Alex M | October 27, 2008 11:25 AM
There are 26 Roman letters and 24 Greek letters, giving us a total of 50, not to mention we have two cases for 100 different letters. I've always wondered why we don't use more of them.
Posted by: C. Chu | October 27, 2008 11:46 AM
In observational astronomy, K is the photometric correction for an object's redshift (your filter passband corresponds to a different part of the object's rest-frame spectrum, depending on its redshift).
Posted by: mph | October 27, 2008 11:51 AM
Kt is a DC motor torque constant
Kv is a DC motor voltage constant
In the SI system these numbers are also THE SAME.
Volts/rad/sec = Newton Meters/Amp
Rather cool and useful and a good slap around the head for those people who want to use imperial units.
Posted by: Chris P | October 27, 2008 12:13 PM
There is a big sign in my high school physics classroom that says "We speak mks here!" I call it the alphabet of physics but it is really the alphabet of science, the SI system. As confusing as the letters for "quantities" is there are letters for the units of all these quantities. And some of the unit letters are the same as other quantity letters. The normal force is often represented by a capital N as a quantity and is measured in newtons, also a capital N. Another confusing one is the symbol for impulse, capital J. But this is the symbol for the unit of energy the Joule also a capital J. There are many others like this. I give quizzes at various times throughout the year aksing questions about quantities and units to see how much they have learned about the alphabet of Physics.
Scientists speak a language that takes a long time to learn. Each letter or group of letters has potentially very far reaching implications. Young students find this very daunting.
Posted by: John-Michael Caldaro | October 27, 2008 12:46 PM
There's a couple more uses of k in the analytical chemistry world that I'm presently blanking on, but will post when I have a chance to go through my class notes and find them.
Posted by: Sarah | October 27, 2008 1:39 PM
There are 26 Roman letters and 24 Greek letters, giving us a total of 50, not to mention we have two cases for 100 different letters. I've always wondered why we don't use more of them.
Not all of them are distinct. Capital Greek letters alpha, beta, epsilon, zeta, eta, iota, kappa, mu, nu, omicron, rho, tau, and chi are identical to capital Latin letters A, B, E, Z, H, I, K, M, N, O, P, T, and X, respectively. Lower case Greek letter omicron is identical to lower case Latin letter O (which is why TeX does not have a command for producing omicron). Also there is no way of reliably distinguishing a handwritten letter O (either case) from number 0; thus capital O is used only to denote order and lower case O is not used at all.
That leaves 85 characters. I wouldn't swear that I have seen somebody use a capital xi, but I have seen all of the others used one way or another. I have also seen the Hebrew letter aleph used (to note a transfinite number).
Posted by: Eric Lund | October 27, 2008 1:42 PM
KD is the dissociation constant for a ligand-receptor complex in boichem. [hormone-hormone receptor, neurotransmitter-receptor etc.]
Posted by: natural cynic | October 27, 2008 1:49 PM
K = Kip
Posted by: Anonymous | October 27, 2008 2:18 PM
I've got to get out Shigley, the god-textbook of mechanical engineering for this, but:
In determining the endurance limit (ie. fatigue strength) of a material or part, the equation goes:
Se = kakbkckdkekfSe'
Of the 'k's:
a = Surface factor
b = Size factor
c = Loading factor
d = Temperature factor
e = Reliability factor
f = Miscellaneous factor (I shit you not).
K = Stress concentration factor for various loading conditions; it has various subscripts to tell them apart.
Kk = Fracture toughness (yes, that's K sub k)
k2 = Radius of gyration
In 2003 the American Gear Manufacturers Association standards for gear design also used 'K' with various subscripts for:
Stress correction, inertia factor, stress cycle factor, load distribution factor, overload factor, reliability factor, size factor, service factor, temperatre factor, dynamic factor, and the lengthwise curvature factor.
I'm sure there are others.
Posted by: Anonymous | October 27, 2008 4:50 PM
Wait, nobody has said K is for Potassium? And before you tell me that's chemistry, not physics, I would argue that chemistry is physics, at a small scale.
Posted by: teawithbuzz | October 27, 2008 5:03 PM
Loads of atomic physicists use potassium.
Posted by: dr. pablito | October 27, 2008 5:30 PM
And of course, the k shell -- the innermost electron shell of atoms, whence k shell electron capture, k shell x-rays. And for wavenumbers, it's an abbreviation for Kaysers, inverse wavelength. Not often used anymore, but sometimes. And it's used in 9-j symbols as another angular momentum vector.
Posted by: dr. pablito | October 27, 2008 5:37 PM
Ka, dissociation constant of an acid (also chem, I guess)
Kb, dissociation constant of a base
Keq, equilibirum constant
Posted by: Leukocyte | October 27, 2008 5:58 PM
The reason why capital xi might not be used is that it could be mistaken for the identical to sign when hand-written.
Posted by: erik Remkus | October 27, 2008 6:11 PM
The ebullioscopic (Kb) and cryoscopic (Kf) constants used in calculating boiling point elevation and freezing point depression, respectively.
Posted by: sangfroid | October 27, 2008 6:28 PM
I seem to remember k_(greek letters, starting from alpha) being used to refer to various spectral lines.
Posted by: Chris | October 27, 2008 9:10 PM
I figured someone would have already mentioned that k is also used generally as a rate constant, especially in first-order decay, i.e., d(WIDE)/dt = -kWIDE, where WIDE stands for Whatever Is Decaying Exponentially.
Posted by: Taxorgian | October 28, 2008 10:14 AM
In population biology, the letter "K" refers to the carrying capacity of a population.
Posted by: Jeremy Mohn | October 28, 2008 12:34 PM
I used capital xi in my dissertation, mostly for the heck of it. Nobody complained.
Posted by: Bob Hawkins | October 28, 2008 2:46 PM
lol, this post reminds me of K-theory
But in all seriousness, some are just typical abbreviations for units. For the constants, it wouldn't surprise me if the reasoning has to do with how 'C', when written fast, can morph into all sort of different funny looking shapes, whereas k just looks like...k
Now, it is always entertaining when solving a multi quantum well problem a textbook uses not only k's but kappas. I call bs.
Posted by: Shawn Wilkinson | October 29, 2008 5:55 AM
k is the 3rd quaternion (I think this is actually the origin of the use of k-hat as the symbol for the z-axis unit vector).
Posted by: andy.s | October 29, 2008 11:28 AM